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Transcript
Brushless
Alternators
Graig Pearen
©2003 Graig Pearen
68
The author prepares to repair a Dunlite BL, 2 KW wind generator
that uses a brushless, 3-phase alternator.
S
everal different technologies are used in
the generator portion of wind turbines
(wind generators). One of the older, more
reliable technologies is the brushless DC alternator.
Its operation is often poorly understood by the
owners, and in some cases, poorly described by the
manufacturers. I’m not an expert on brushless
alternators, but when I acquired some damaged
Dunlite wind turbines, I became seriously interested in how they work.
home power 97 / october & november 2003
alternator technology
Basic Theory
When an electric current is passed through a coil of wire,
a magnetic field is produced (an electromagnet). Conversely,
when a magnetic field is moved through a coil of wire, an
electric current is produced in the wire. Both of these actions
take place in alternators, motors, and generators or
dynamos.
The stationary part of a motor or alternator is called the
stator and the rotating part is called the rotor. The coils of
wire that are used to produce a magnetic field are called the
field, and the coils that produce electricity are called the
armature. The coils of wire are sometimes referred to as the
“windings.”
Electrical energy is generated when a coil of wire is
moved through a magnetic field. It doesn’t matter whether
the coil is moving or the magnetic field is moving. Either
configuration works equally well, and both are used
separately or in combination depending on mechanical,
electrical, and other objectives. The old DC generators
(dynamos) used a stationary field and rotating armature.
Automotive alternators use the opposite configuration—a
rotating field and stationary armature. In a brushless
alternator, both configurations are used in one machine.
Alternator
Terminology
The parts of an alternator or related equipment
can be expressed in either mechanical terms or
electrical terms. These two sets of terminology are
frequently used interchangeably or in combinations that include one mechanical term and one
electrical term. This causes great confusion when
working with compound machines such as a
brushless alternator, or when conversing with
people who are used to working on a machine that
is configured differently than the machines that
the speaker is used to.
Mechanical
Rotor: The rotating part of an alternator, generator,
dynamo, or motor.
Stator: The stationary part of an alternator,
generator, dynamo, or motor.
Construction
Electrical
A brushless alternator is composed of two alternators
built end-to-end on one shaft. Smaller brushless alternators
may look like one unit, but the two parts are readily
identifiable on the large versions. The larger of the two
sections is the main alternator and the smaller one is the
exciter.
The exciter has stationary field coils and a rotating
armature (power coils). The main alternator uses the
opposite configuration with a rotating field and stationary
armature. This configuration is required to implement
brushless technology without using permanent magnets.
Eliminating the brushes eliminates brush and slip ring wear
and maintenance. The only moving parts subject to wear in
the basic alternator are the bearings.
Armature: The power-producing component of an
alternator, generator, dynamo, or motor. The
armature can be on either the rotor or the stator.
Brushless Alternator
Configuration
Field: The magnetic field component of an
alternator, generator, dynamo, or motor. The field
can be on either the rotor or the stator, and can be
either an electromagnet or a permanent magnet.
Although rugged and reliable, brushless alternators are
expensive to manufacture and therefore were only used in
premium quality wind turbines. The high cost of production
puts them solidly in the industrial grade category. With the
advent of powerful, cheap permanent magnets, the
residential-sized wind industry has switched to the cheaper
permanent magnet alternator (PMA) technology. Brushless
alternators are also used extensively in large engine-driven
power plants and in utility-sized power generation.
Exciter Field
Exciter Armature
Exciter Field
Diode Mounting Plate
Main Armature
Exciter
Main Field
Main Armature
Rotor
Stator
Shaft
The exciter field coils are on the stator, and its armature
is on the rotor. The AC output from the exciter armature is
fed through a set of diodes that are also mounted on the
rotor to produce a DC voltage. This is fed directly to the field
coils of the main alternator, which are also located on the
rotor. With this arrangement, brushes and slip rings are not
required to feed a current to the rotating field coils. This can
be contrasted with a simple automotive alternator, where
brushes and slip rings are used to supply a current to the
rotating field.
www.homepower.com
69
alternator technology
The rotor of a brushless alternator: The smaller section at left is the exciter armature; the larger section at right is the main alternator
field. The diodes are mounted on the aluminum disk at the left end.
Main Alternator
The main alternator has a rotating field as described
above and a stationary armature (power generation
windings). With the armature in the stationary portion of
the alternator, the high current output does not have to go
through brushes and slip rings. Although the electrical
design is more complex, it results in a very reliable
alternator because the only parts subject to wear are the
bearings.
Control System
Varying the current through the stationary exciter field
coils controls the strength of the magnetic field in the exciter.
This in turn controls the output from the exciter. The exciter
output is fed into the rotating field of the main alternator to
supply the magnetic field for it. The strength of the magnetic
field in the main alternator then controls its output.
The result of all this is that a small current in the field of
the exciter indirectly controls the output of the main
alternator, and none of it has to go through brushes and slip
rings.
Automotive Alternator
Positive
Stator:
Armature
Rotor:
Field
Bridge
Rectifier:
Six Diodes
Negative
Automatic Voltage Regulator
An automatic voltage regulator (AVR) serves the same
function as the voltage regulator in an automobile or the
charge controller in a solar-electric system. It measures the
battery voltage and uses that information to adjust the level
of current fed into the field coil. As the battery voltage goes
down, the AVR increases the current to the field coils to
produce a stronger magnetic field.
The stronger magnetic field causes a higher output from
the alternator to bring the battery voltage back up to the set
point. Conversely, when the voltage rises too high, the field
current is decreased, lowering the output from the
alternator.
Variations
As with any other electronic or electrical device, there
can be numerous variations in the design. The Dunlite 2 KW
wind machines use an auxiliary winding on the main stator
to generate the voltage for the exciter field. These machines
rely on residual magnetism to excite the auxiliary winding,
while some machines use permanent magnets for this
purpose.
On the Dunlite machines, both the exciter and main field
coils use eight poles. (Three phases, multiplied by eight
poles, is twenty-four coils of wire.) The exciter armature is
wound in a three-phase, four-wire wye (star) configuration,
and the main armature is a three-phase, three-wire delta
design.
Three-Phase Basics
Slip Rings
& Brushes
Voltage
Regulator:
Automatic
70
In a wind generator, this means that the loading can be
matched to the speed, unlike in a permanent magnet
alternator. Although some of the wind generator’s output is
used to generate the magnetic field, there is no cogging
effect, so low wind start-up characteristics are improved.
A three-phase alternator has a minimum of three sets of
windings, spaced 120 degrees apart around the stationary
armature (stator). As a result, there are three outputs from
home power 97 / october & november 2003
alternator technology
Brushless, 3-Phase Alternator
Positive
Rotor:
Exciter Armature
Stator:
Exciter Field
Stator:
Main Armature
Bridge
Rectifier:
Six Diodes
Bridge
Rectifier:
Six Diodes
Rotor:
Main Field
Negative
Voltage
Regulator:
Automatic
the alternator, and they are electrically spaced 120 degrees
out of phase with each other. A multi-pole design will have
multiple sets of three windings. The more poles there are,
the slower the alternator turns for a given voltage and
frequency.
More poles increase the complexity of the alternator, and
that in part accounts for the higher price of slow speed
versions. Other than in single-phase power plants, most
alternators, including the automotive type, generate threephase power. A three-phase AC alternator will not have any
diodes in it.
If the output is DC, it will probably have six diodes to
convert the output from the main alternator to DC. This is
the configuration used in automotive alternators. A threephase brushless alternator may have four or six diodes on
the rotor for the exciter output in addition to the diodes that
may be on the stator.
There are two ways that three-phase machines can be
wired. One is the delta (triangle) configuration, with one
wire coming off each “point of the triangle.” The other is the
wye (Y) or star configuration. They have one wire from each
branch of the Y and a fourth common wire is added from the
center point of the Y (the common connection point between
the windings). Multiple voltage machines will have
additional wires to allow them to be configured for the
desired system voltage.
If you ever have to service an old wind turbine with a
brushless alternator, this brief description may help you to
understand how it works, or at least let you talk the same
language as the repair shop.
Access
Graig Pearen • [email protected] •
www3.telus.net/pearen
See “Yer Basic Alternator,”
Bob-O Schultze, HP20 , available in print or on the Solar2 CD.
www.homepower.com
71